1. Field of the Invention
The invention relates to a technology for transforming combustible materials into clean and highly efficient synthetic gas, and more particularly to a method and a system for producing synthetic gas from biomass by low temperature pyrolysis and high temperature gasification.
2. Description of the Related Art
A typical biomass gasification technology includes: fixed bed gasification, fluidized bed gasification, and two stages gasification, all of which are direct gasification technologies. The processes of direct gasification technologies are characterized in that the heat produced by part of the biomass supplies energy resource for gasification, the air, oxygenized air, or a combination of the oxygenized air and water vapor is functioned as an oxidant during the gasification reaction. However, studies have shown that technologies of direct gasification of the biomass are disadvantageous in the following aspects:                First, the components and the heat value of the biomass fuels are unstable, the biomass has low fire point and fast combustible reaction, thus, explosion easily occurs. When part of regions are superheated and coked, the operating temperature of the gasifier is very difficult to control.        Second, when the air works as an oxidant in which the content of the inactive gas of N2 is prominent, it results in a higher content of N2, a lower content of effective gas (CO+H2), and a lower ratio of H2/CO, besides, the heat value of the synthetic gas is low and unstable, which only maintains at 5000 KJ/Nm3 below and hardly meets the need of the later industrial utilization.        Third, when the oxygenized air works as an oxidant, although the content of N2 is relatively lowered, an additional air separating device is necessitated. Because of a large capacity and high energy consumption of the air separating device, such a process largely increases the production cost.        Fourth, when the oxygenized air and the water vapor work as both oxidants, although the content of N2 in the synthetic gas is lowered, and the content of H2 is increased, the water vapor working as a reacting medium still consumes a large amount of heat energy, plus the energy consumption in the air separation, the process largely maximizes the production cost.        Fifth, about 15-20% of the biomass is necessitated to self-ignite for providing the energy resource for gasification, but at the same time a large amount of CO2 is produced in the combustion, correspondingly, the content of effective gas (CO+H2) is decreased. Furthermore, the high temperature synthetic gas and the mixed air carry a large amount of sensible heat, and thus, the conversion of the heat energy into the chemical energy is largely minimized, and the efficiency of the cooled gas is also lowered, which is generally 70% below and no higher than 80% in exceptional conditions.        Sixth, the operating temperature of the gasifier is generally controlled at 800-1200° C., at such a temperature, the gasification of the biomass produces a large amount of tar which is difficult to remove, and too much of tar aggregated in the device and pipes is apt to cause pipe blocking and device contamination.        Seventh, the gash produced in the gasification of the biomass contains a prominent content of alkali metal oxides comprising K and Na, which is general 20-40 wt. % of the total ash. However, at a temperature higher than 800° C., the alkali metal oxides is apt to be gasified and mixed into the synthetic gas, which not only affects the property of the synthetic gas, but also adheres to the pipes and devices together with the tar, thereby resulting a serious corrosion on the devices and pipes.        
In view of the above existing problems, technologies of direct gasification of biomass are difficult to be applied in practical production. Thus, a method for gasifying the biomass which can be applied in industrial production and converted to commercial benefits is desired.